1. Field of the Invention
The present invention relates to a substrate for crystal growth forming semiconductor layer, a semiconductor device such as, for example, but not limited to, a power device and an optical device, and a manufacturing method of a semiconductor device, and more particularly to the substrate, the semiconductor device, and the manufacturing method for forming a III-V group compound semiconductor layer on the surface of the substrate and for example, manufacturing a high-output high frequency transistor.
2. Description of the Related Art
As a material of a high-output power device operating at high frequency band, various III-V group compound semiconductors are used. Among them, in recent years, nitride semiconductors such as gallium nitride (GaN) and aluminum gallium nitride (AlGaN) having high break down voltage and capable of operating at a high voltage are used as a material for high-output frequency devices.
Such a nitride semiconductor is grown on the substrate or a semiconductor layer or an insulating layer formed on the substrate by epitaxial growth using the MOCVD (metal-organic chemical vapor deposition) method or MVD (molecular vapor deposition) method. In order to realize growth of a stable semiconductor crystal, it is desirable that the growth layer and substrate have the same lattice constant and coefficient of thermal expansion. However, it is actually difficult to obtain such a substrate, so that for the substrate as a sapphire substrate and a SiC substrate which are comparatively inexpensive, have a larger size wafer, and are different in the lattice constant and coefficient of thermal expansion are often used.
However, when a nitride semiconductor layer such as GaN or AlGaN is grown on a sapphire substrate, the substances have large mismatch and coefficient of thermal expansion. Therefore, due to the thermal strain between the substrate and the semiconductor layer formed on the substrate, the substrate may be warped and irregular cracks may be occurred in the semiconductor layer.
For example, a field effect power device for amplifying high frequency power is fabricated using such a substrate. A plurality of elements are formed on the substrate, through a plurality of processes such as a deposition of semiconductor layers and insulating layers and a process for photolithography.
During these processes, when fixing the substrate to the base using such as a vacuum chuck method, it is difficult to hold the substrate flat due to the aforementioned warp of the substrate. For example, at the process for deposition, the temperature distribution of the substrate becomes non-uniform, at the process for photolithography, uniform exposure is difficult. Therefore, in order to obtain a good yield, through all the manufacturing processes, it is required to suppress, as much as possible, an occurrence of deformation such as a warp without applying stress to the substrate.
To suppress an occurrence of such a warp or irregular cracks of the sapphire substrate, for example, in Japanese Patent Publication 2003-113000, an art as indicated on Page 4 and in FIG. 1 for providing a stress offset layer on the back side of a substrate and giving beforehand tensile stress to the substrate is disclosed.
However, by this method, stress is applied to the substrate to suppress the warp. Therefore, through all the processes for manufacturing the power device, it is difficult to continue good suppress of the warp of the substrate. Further, when a crack is caused once to the formed semiconductor layer, the crack is distributed irregularly in the semiconductor layer. Therefore, a problem arises that the reliability and yield of a plurality of power devices formed from this substrate are lowered unexpectedly.
Further, for example, in Japanese Patent Publication H11-40849, a method for installing a destruction induction section composed of a notch groove on the bottom of the sapphire substrate and forming a mask layer at the position on the top of the sapphire substrate corresponding to the destruction induction section is disclosed in the item of and FIG. 1 therein.
However, according to this method, there is a fear that a problem may be caused to the processes. Generally, the thickness of a GaN layer formed on the sapphire substrate is several μm or so, though the sapphire substrate is very thick such as several hundreds μm, that is, several hundreds time of it. Therefore, forming itself of a cracked face extending from the bottom to the top on the very thick sapphire substrate is expected to be greatly hard.